An interesting polemic! I'll express my opinion as well. I believe that Earth and Air are the most necessary magics. It is impossible to say definitively which one is better. As many have already noted, it is impossible to get by without Slow, Restore, Town Portal, Haste, Dimension Door, and Fly (by the way, for some reason no one mentioned same-//Destruction Beam—a very specific tool when you have Blindness and a good mana reserve, and are fighting against a large number of strong monsters). Well, if I really have to choose, I lean more towards Earth, because it would be much harder to play without Town Portal, Restore, and Slow than without Dimension Door and Fly.
And of course, one should not forget about Water magic, especially if you are playing as Archangels or there are many of them in the army and you have Clone. I also consider spells such as Bless, Prayer, Heal, Remove Spell, and Forgetfulness to be very useful.
As for Fire magic, I don't consider it necessary; although Berserk is certainly a great thing, I've already forgotten when I last used it.